Today, smart cards are widely used in society. For example, smart cards may be used as electronic identity (eID) cards. The end-user acceptance of such eID cards, however, is still relatively low. Although the eID cards are relatively secure, due to their cryptographic capabilities, they are usually not equipped with a user interface suitable for entering user credentials, i.e. a so-called “authentication interface”. As a consequence, the majority of eID users still enter their PIN code through personal computers and laptops, which increases the risk that their credentials are intercepted by malicious software such as Trojans and key-logger programs.
It is known to integrate an authentication interface into a smart card. For example, EP 2 667 156 A1 describes a capacitive position sensor system for determining the position of an object, wherein the object is positioned within a sensitive area of the capacitive position sensor system and changes the capacitance of capacitors being arranged underneath the object. The capacitive position sensor system comprises a first plurality of sensing elements, each sensing element comprising a first capacitor having a first electrode and a second electrode, wherein each first electrode is coupled via a switch to a voltage supply to form a switched capacitor filter, wherein the second electrodes are coupled to form a sensing line, a sensing unit, wherein the sensing unit is adapted to sense a voltage level representing the amount of charge being present on the sensing line, and a control unit, wherein the control unit is adapted to apply a drive signal to each of the switches being coupled to the first electrodes. In one integration cycle, a part of the switches being coupled to the first electrodes is closed so that a part of the first capacitors is driven by a first drive signal, wherein the sensing unit is adapted to sense the voltage level representing the sum of the amount of charge of the part of the first capacitors, wherein the number of the switches being closed is at least two. The control unit is adapted to determine the position of the object by analyzing the results of a plurality of sensed voltage levels of a plurality of integration cycles. This capacitive position sensor system is an example of a touch-based user interface that may be embedded into a smart card.
It is still relatively difficult to fabricate a multi-functional eID card with an embedded authentication interface. For instance, it is typically necessary to use double-sided inlays requiring VIAs (i.e. contacts between both layers) and to use a large number of sensor terminals. It might be desirable to use a smaller number of sensor terminals, which in turn would require less interface connections to a processing module, thereby reducing cost. Reducing the number of sensor terminals may be achieved by reducing the number of sensors embedded in the authentication interface. For example, the authentication interface may be based on a relatively simple 2×2 sensor array, as shown in FIG. 1A. However, in that case the usable area of the authentication interface is relatively small, as shown in FIG. 1B. Therefore, a conflict seems to exist between the manufacturing requirements and the performance requirements of such interfaces.
FIGS. 1A and 1B show a conventional user interface unit. FIG. 1A shows a user interface unit 100 which comprises a plurality of capacitive sensors 102a, 102b, 102c, 102d. The capacitive sensors 102a, 102b, 102c, 102d are arranged in an array that has a size of 2 by 2 sensors. FIG. 1B shows a usable area 104 of the user interface unit 100 shown in FIG. 1A.
In such user interface units, the position of an object, for example a user's finger, is typically obtained by applying a center-of-gravity calculation to the activity levels determined for the individual sensors in the sensor arrays. However, the outer sensors exhibit a dead-zone at their perimeter where a change in position will not result in a change of the measured activity level. Typically, the size of the dead-zone in each dimension is the size of the sensor in that dimension minus the size of the finger. When applying a weighting function such as a center-of-gravity formula the resulting position is only valid inside apart of the total sensor area. The area in which a resulting position is valid is referred to as the “usable area” herein. The usable area is the total area covered by all sensors reduced by the dead-zone areas of all sensors, as indicated by the black dots in FIG. 1B. For a 2×2-sensor array, as shown in FIG. 1B, the usable area may have a size of ¼ of the total size of the sensor array. For a 3×3 sensor array the usable area may typically have a size of 4/9 of the total size of the sensor array. More sensors typically result in a larger relative usable area. For example, for a 4×4 sensor the usable area may have a size of 9/16 of the total size of the sensor array.
However, an increased number of sensors results in more sensor signals, which typically requires a longer capturing and processing time and hence requires more energy. Furthermore, in case of an array-like sensor structure having a size of 3 or more in each dimension, the inner sensors may not be contacted without signal crossings or VIAs. As a consequence, two-layer inlays with VIAs may be required, which may increase the manufacturing cost and reduce the user interface's reliability. Furthermore, sensor structures that require double-sided inlays may not support economic inlay manufacturing such as by silver-ink printing, which is typically requested by some smart card manufacturers.